In recent years, in order to satisfy social needs such as protection of environment and nature, researches have been actively conducted on fuel cells which are efficient and clean energy sources. Among them, polymer electrolyte fuel cells (PEFC) and hydrogen internal combustion engines are expected as energy sources for homes, vehicles, etc. because they can operate at low temperature and are high in output density.
In these systems, detection of gas leakage is important because these systems use hydrogen which is a combustible gas as fuel.
There has been known a combustible gas detection device which detects the concentration of such a combustible gas present in an object atmosphere. The known combustible gas detection device includes a gas detection element which is disposed in an object atmosphere and which includes a heat generation resistor and a temperature measurement resistor provided thereon. The resistance of the heat generation resistor changes due to a change in the temperature of the heat generation resistor itself (due to heat generation of the heat generation resistor). The resistance of the temperature measurement resistor changes due to a change in the environmental temperature.
Specifically, in this combustible gas detection device, the resistance of the heat generation resistor in the gas detection element is controlled through use of a bridge circuit such that the resistance alternately becomes equal to resistances corresponding to two set temperature (first and second set temperatures), and the concentration of the combustible gas is calculated from control voltages (voltages across the heat generation resistor) at that time and a voltage difference (temperature voltage) produced as a result of a change in the resistance of the temperature measurement resistor.
The switching between the two set temperatures (first and second set temperatures) is performed, for example, by alternately selecting fixed resistors which are provided in the bridge circuit and have different resistances every time a predetermined period of time elapses (see, for example, Japanese Patent No. 4302611).
Incidentally, with recent progress of the integration technique, the degree of miniaturization of gas detection elements increases. As a result, in the combustible gas detection device, the heat generation resistor and the temperature measurement resistor tend to be disposed closer to each other as compared with conventional gas detection elements.
In the conventional combustible gas detection device, the influence of a change in the heat generation temperature of the heat generation resistor (i.e., the difference between the first set temperature and the second set temperature) on the temperature measurement resistor is not considered. Therefore, because of this influence, an error is produced between the actual environmental temperature and the environmental temperature determined on the basis of the temperature voltage of the temperature measurement resistor, whereby the accuracy in detecting the gas concentration decreases.